Method of using polyethylene terephthalate as a bonding adhesive

ABSTRACT

A method of bonding a sheet or film of polyethylene terephthalate to another sheet of polyethylene terephthalate or in employing a sheet of polyethylene terephthalate as a high strength adhesive for bonding materials together wherein the sheet or sheets of polyethylene terephthalate are placed in pressure contact with each other or in pressure contact with materials to be bonded together, in a continuously evacuated environment and are heated to a temperature sufficient to out gas impurities from the polyethylene terephthalate sheets and fuse the sheets to the adjacent surface.

United States Patent Frankowski 51 May 9,1972

[54] METHOD OF USING POLYETHYLENE TEREPHTHALATE AS A BONDING ADHESIVE[72] Inventor: Jozef Frankowski, La Jolla, Calif.

[73] Assignee: General Dynamics Corporation, San

Diego, Calif.

22 Filed: Oct. 30, 1968 [21] Appl.No.: 771,902

[52] US. Cl ..l56/285, 156/306 [51] Int. Cl ..B29c 17/00 [58] Field ofSearch 156/285, 306, 322, 382

[56] References Cited UNITED STATES PATENTS 2,719,100 9/1955 Banigan156/306 X 2,786,778 3/1957 Palmquist ....156/332 X 3,000,775 9/1961 Shawet a1.. ....l56/332 X 3,054,703 9/1962 Brasure 156/332 X LECTRICAL3,078,201 2/1963 Christie .156/306 X 3,188,265 5/1965 Charbonneau.156/306 X 3,198,686 8/1965 Caligari 156/332 X 3,372,148 3/1968 Wiener......l56/332 X 3,493,451 2/1970 Beery ..l56/382 X PrimaryE.\'aminerBenjamin R. Padgett Assistant Examiner-Gary G. SolystAttorney-Neil F. Martin and Carl R. Brown [5 7] ABSTRACT A method ofbonding a sheet or film of polyethylene terephthalate to another sheetof polyethylene terephthalate or in employing a sheet of polyethyleneterephthalate as a high strength adhesive for bonding materials togetherwherein the sheet or sheets of polyethylene terephthalate are placed ina pressure contact with each other or in pressure contact with materialsto be bonded together, in a continuously evacuated environment and areheated to a temperature sufficient to out gas impurities from thepolyethylene terephthalate sheets and fuse the sheets to the adjacentsurface.

3 Claims, 4 Drawing Figures Patented May 9, 1972 48 CLEAN SURFACES 0FPOLYETHYLENE TEREPHTHALATE FILM a METAL SURFACES PLACE POLYETHYLENETEREPHTHALATE SURFACES IN CONTACT 8 COVER OUTER SURFACES WITH GLASSCLOTH IN VACUUM LECTRICAL APPLY PRESSUREG HEAT 56 PLACE POLYETHYLENE 58TEREPHTHALATE FILM ON METAL SURFACES TO BE BONDED lH EAT IN VACUUM[gamma 8. COOL I SEMBLE BOND EAT IN VACUUM 1- 66 JOZEF (NMI) FRANKOWSKIATTORNEY METHOD OF USING POLYETHYLENE TEREPl-ITHALATE AS A BONDINGADHESIVE BACKGROUND OF THE. INVENTION There is a need for a highstrength adhesive for bonding metal to metal, metal to glass, metal to aceramic, metal to reinforced materials, such as laminates, and also forbonding dissimilar materials such as stainless steel to titanium,stainless or aluminum. There is a particular need for such an adhesivethat can maintain a high strength bond at elevated and cyrogenictemperatures and for providing multilayer insulation on space propulsionstages that are subject to both elevated temperatures and to cyrogenictemperatures. While sheets or layer of polyethylene terephthalate,better known by the trade name Mylar, are used for multilayer insulationon space propulsion stages, it has been necessary to use other adhesivesto bond the sheets of Mylar to the space propulsion stages. It has beenfound that the melting temperature and the degregation temperature oroxidation temperature of Mylar at ambient pressure are very closetogether, and the out gassed material during Mylar heating condenses onthe surfaces of the Mylar sheets or on the surfaces of the adherendsmaking it impossible to provide a good bond between the Mylar sheets orto employ Mylar sheets as an adhesive. While the Mylar sheets haverelatively high strength at elevated and cyrogenic temperatures, theadhesives often break down at such temperature ranges. Further it ismore expensive and requires added complexity to use known adhesives forjoining Mylar sheets to other Mylar sheets or to adhering Mylar sheetsto other materials.

Thus it is advantageous to be able to bond polyethylene terephthalate orMylar sheets to other Mylar sheets or Mylar sheets to other materialswith a high strength bond employing Mylar sheets as the sole bonding oradhesive material without requiring other adhesives.

SUMMARY OF THE INVENTION In the methods of this invention, polyethyleneterephthalate or Mylar sheets or films are applied to other sheets orfilms of Mylar in a vacuum that is continuously evacuated by a suitablevacuum pump. The abutting films are enclosed with a porous material,such as glass cloth, that has a melting temperature higher than that ofMylar. A low magnitude pressure is applied to opposite surfaces of theglass cloth and Mylar films and the environmentis heated for ashortperiod of time to a temperature sufficient to out gas impuritiesfrom the polyethylene terephthalate or Mylar sheets. The two layers ofMylar sheets are fused together under the temperature and pressure andthe out gassed impurities that otherwise would condense on the surfacesof the Mylar sheets and destroy the bond therebetween are removed by thevacuum pump.

In other embodiments of the invention, a polyethylene terephthalate orMylar film is applied to the smooth surface of a material, such asmetal, glass, ceramic, reinforced materials, or other suitable materialsin a vacuum that is continuously evacuated. A porous film of, forexample, glass cloth, is laid on the outer surface of the Mylar sheetand a slight pressure is applied to the film holding the sheet incontact with the material. Again the environment is heated to atemperature sufficiently high to out gas impurities from the Mylar sheetand fuse the sheet to the surface of the material. The same process isemployed to bond a Mylar sheet to the smooth surface of a secondmaterial. Then the bonded Mylar surfaces of the two materials are joinedtogether in the vacuum environment and under contact pressure. Thebonded sheets are heated to the fusing temperature that may be in theorder of 500 to 650 Fahrenheit for at least seconds, wherein thepolyethylene terephthalate bonding fuses together providing a highstrength adhesive bond that joins the materials together.

The polyethylene terephthalate sheet may comprise type A Mylar made byDuPont Corporation. The minimum applied pressure during the bonding stepis about 1 X 10 Torr and the out gassing temperature is about 550 to 670Fahrenheit for a time period of 3 to 30 seconds, depending upon thethickness of the Mylar sheets fused. Generally the Mylar sheets have athickness of about 0.005 of an inch. lt has been found that the highstrength bond is maintained at temperatures ranging from about -423 to400 Fahrenheit.

It is therefore an object of this invention to provide a new andimproved method of bonding sheets of polyethylene terephthalate.

It is another object of this invention to provide a new and improvedmethod for employing sheets of polyethylene terephthalate as a highstrength adhesive for bonding metal to metal, metal to glass, metal toceramiqmetal to reinforced materials, and for bonding dissimilar metals.

It is another object of this invention to provide a new and improvedmethod for bonding sheets of polyethylene terephthalate or for employingsheets of polyethylene terephthalate as a high strength adhesive thatpermits out gassing of impurities from the polyethylene terephthalatesheets and that fuses the sheets to the adjacent surfaces to be bonded.

Other object and many attended advantages will become more apparent upona reading of the following detailed specification and an examination ofthe drawings wherein like reference numerals designate like partsthroughout and in which:

FIG. 1 is an illustration in block form of steps in the method of thisinvention.

FIG. 2 is a schematic view of a vacuum oven chamber having Mylar sheetspositioned for being bonded together.

FIG. 3 is a cross sectional view of sheets of metal that are bondedtogether with a Mylar sheet.

FIG. 4 is a cross sectional view of a Mylar sheet bonded to a metalsheet.

A polyethylene terephthalate sheet, such as Mylar made by the DuPontCorporation, is employed to bond sheets of Mylar to other sheets ofMylar or in bonding Mylar to the surface of another material, or as ahigh strength adhesive for bonding materials to other materials. TheMylar sheets are initially cleaned by wiping with a lint free cloth thatmay be moistened with acetone or other suitable cleaners and that isallowed to dry in air. The surfaces of metal sheets or sheets of othermaterials may be cleaned in any suitable manner, depending upon thematerial. In the method of joining polyethylene terephthalate sheets orMylar sheets 30 and 32, see FIG. 2, the sheets are joined together witha layer of glass cloth 34 laid on the outer surface of the abuttingsheets. The glass cloth 34 and abutting sheets are laid on a rigidsurface 28 and an over plate 36 is laid on the upper surface of theglass cloth 34. The assembly is positioned in an evacuated environment12 that is evacuated by a suitable vacuum pump 18 that draws gasesthrough input conduit 16 and ejects the gases through output line 20.The housing 10 of the vacuum chamber comprises an evacuated oven havinga top surface 22 with an insulated heating coil 24 that passestherethrough. An electric power source 26 supplies power to the heatingcoil 24 that heats the chamber 10 and thus raises the temperature in thespace 12 to the required temperature for joining the polyethyleneterephthalate sheets. It should be understood that only a representativeshowing of an enclosed, evacuated oven 10 is illustrated in FIG. 2 andother suitable vacuum ovens may be employed. 1

The temperature in vacuum 12 is raised to heat the respective Mylarsheets to a temperature of about 550 to 670 Fahrenheit for a period oftime of approximately 3 to 30 seconds. This temperature causesimpurities in the commercial grade Mylar to out gas in the form of a gasdischarge. This gas is drawn by vacuum pump 18 through the porous glasscloth 34 and is pumped to the outer atmosphere through conduit 20. Thisremoves the out gassed impurities from the bonding surface of the Mylarsheets and upon cooling the Mylar sheets are fused together in a highstrength bond. The same process is employed to bond a Mylar sheet to asheet of material as illustrated in FIG. 4, with the exception that theglass cloth is only laid on the Mylar side.

It has been found that between the temperatures of 550 to 670Fahrenheit, the polyethylene terephthalate sheets become hot enough toflow but do not melt. The polyethylene terephthalate sheet adheres tothe adjacent polyethylene terephthalate sheet and does not adhere to theglass cloth. The pressure applied to the joined sheets need not belarge. However, the pressure should be enough to obtain intimate contactbetween the adjacent surfaces of the sheets. It has been found byexperimentation that if the temperature is lower than 550 Fahrenheit,then there is insufficient out gassing of the impurities and a poor bondresults. If the temperature is above about 670 Fahrenheit, the Mylarfilm darkens because the degradation temperature of the Mylar film hasbeen exceeded. For Mylar sheets of the thickness of 0.005 or less, atemperature time of 3 seconds will normally provide sufficient time forout gassing of the impurities to provide a reasonably good bond. Whenthe thickness of the Mylar sheet is increased, the time may extend tothe order of 30 seconds.

It may be understood that the aforesaid process may be repeated tolaminate several layers of polyethylene terephthalate sheets or to applya polyethylene terephthalate sheet surface to other materials. In themethod of applying polyethylene terephthalate sheets to, for example ametal surface, the sheet of metal 44 may be placed on the support member28 in the vacuum oven 10. A polyethylene terephthalate sheet 46 is thenlaid on the upper surface of the metal sheet 44 and a layer of glasscloth is placed on the upper surface of the Mylar sheet 46, and plate 36is placed thereon for the required pressure. The vacuum environment 12is then heated to the temperature sufficient to out gas impurities fromthe polyethylene terephthalate sheet and fuse the polyethyleneterephthalate sheet to the surface of the sheet of metal 44. It may beunderstood that successive layers of polyethylene terephthalate sheetsmay be similarly applied to the other side of the sheet metal 44 or tosuccessive surfaces of polyethylene terephthalate sheets forminglaminate films on the metal surface 44.

To join one material with another material, such as illustrated in FIG.3, a sheet of metal, for example sheet metal 42,

has a sheet 40 of polyethylene terephthalate applied thereto in themanner described relative to FIG. 4. A similar sheet of polyethyleneterephthalate 40 is applied to the surface of a metal sheet 38 in thesame manner as previously described relative to FIG.-4. These twosheets, 38 and 42, are then laid one on top of the other with the bondedpolyethylene terephthalate sheet in abutting position. Contact pressurebetween the adjacent bonded surfaces is obtained by the weight of theupper material, or by such other additional pressure as necessarytoachieve overall contact pressure. After the bond is assembled, therespective sheet elements are placed in a vacuum oven and raised to atemperature of 500 to 650 Fahrenheit for a time period of about 10seconds or more duration, depending upon the thermal properties of thematerials being joined, the heat up time of the materials being joined,the thickness of the sheet materials being joined, and otherrecognizable factors, to achieve a fusing of the polyethyleneterephthalate bonds. Upon cooling, a high strength bond between therespective sheet materials is achieved.

The heating and joining of the respective sheets and materials in thevacuum not only draws out gassed impurities from the heated vacuumenvironment, but also prevents oxidation of bonding surfaces between thejoined materials. The following specific examples willserve to furtherillustrate the practice of the invention.

A pair of sheets of polyethylene terephthalate, type A Du- Pont Mylar,having a thickness of 5 mils, were laid one on top of the other andcovered with glass cloth after being suitably cleaned. Under a pressureof l X 10" Torr, the sheets were heated in a vacuum environment that wascontinuously evacuated for approximately 3 seconds at a temperature of550 Fahrenheit. The joined polyethylene terephthalate sheets wereremoved and the laminated sheets were found to have a high strengthbond.

A pair of polyethylene terephthalate sheets, type A DuPont Mylar, havinga thickness of approximately 0.005 mils, were covered with glass clothand placed in a vacuum environment that was continuously evacuated undera pressure in the order of 2.5 X 10' Torr. The sheets were heated to atemperature of about 665 Fahrenheit for 5 minutes and were then removedand cooled. A high strength bond was obtained.

III

A Mylar film of about 5 mils was applied to a sheet of titanium having athickness of about 0.02 inch at a temperature of about 550 Fahrenheitfor a period of about 2 minutes under a bonding pressure ranging from1.2 X 10 to 2.5 X 10' Torr in a continuously evacuated oven. The joinedmaterial was removed and cooled and found to have a high strength bond.

The bonded stainless steel sheet of the above referenced Experiment IIIand the bonded titanium sheet of the above referenced Experiment IV,were joined together with their polyethylene terephthalate surfacesabutting and were heated to a temperature of 500 Fahrenheit for about2minutes in a vacuum pressure in the order of l X 10' Torr. The joinedmetals were removed and tested and found to have a lap strength of 4,000psi in tensil sheer.

The separate bonded sheets of experiment V above, were joined inabutting relationship and heated to a temperature of 665 Fahrenheit witha bonding pressure varying from 1.8 X 10" to 2.5 X 10 Torr in a vacuumenvironment for a period of 5 minutes. The joined bonded sheets wereremoved and cooled and various sections were tested for tensil strengthand lap strength and found to have an average lap strength of 5,238 psi.

Having described my invention, I now claim:

1. A method of bonding a sheet of polyethylene terephthalate to a secondsheet of material having a melting temperature at least as high aspolyethylene terephthalate, comprising the steps of;

positioning in an evacuable chamber a sandwich assembly comprising asheet of polyethylene terephthalate laid upon a second sheet ofmaterial; said sandwich assembly positioned upon a rigid surface suchthat said second sheet of material is in direct contact with said rigidsurface;

and heating said sandwich assembly to a temperature of from about 550 to670 F. for about at least 3 seconds in a partial-vacuum environment tocause out gassing of impurities and bonding of the polyethyleneterephthalate sheet to the second sheet of material.

2. The method according to claim 1, wherein a porous sheet beingnon-bondable with polyethylene terephthalate at said temperatures ispositioned upon the sandwich assembly in direct contact with thepolyethylene terephthalate sheet and an over-plate is positioned uponsaid porous sheet so as to cause the polyethylene terephthalate sheet tobe in more intimate contact with said second sheet of material.

3. The method according to claim 2, wherein the second sheet of materialis polyethylene terephthalate, and comprising the additional step ofdisposing a porous sheet being non- 5 bondable with polyethyleneterephthalate at said temperatures between the sandwich assembly andsaid rigid surface.

2. The method according to claim 1, wherein a porous sheet beingnon-bondable with polyethylene terephthalate at said temperatures ispositioned upon the sandwich assembly in direct contact with thepolyethylene terephthalate sheet and an over-plate is positioned uponsaid porous sheet so as to cause the polyethylene terephthalate sheet tobe in more intimate contact with said second sheet of material.
 3. Themethod according to claim 2, wherein the second sheet of material ispolyethylene terephthalate, and comprising the additional step ofdisposing a porous sheet being non-bondable with polyethyleneterephthalate at said temperatures between the sandwich assembly andsaid rigid surface.